The present invention relates to mixer circuits for generating intermediate frequency signals that retain the modulation characteristics of an original modulated carrier wave and, more particularly, to balanced diode mixers exhibiting low levels of intermodulation distortion.
Balanced mixers using matched diodes and center-tapped transformers have come into wide spread use as components in VHF and UHF communication systems. Such mixers exhibit low levels of intermodulation distortion and good isolation between the various input and output ports to the mixer circuit. Nevertheless, conventional diode mixers are subject to substantial amounts of intermodulation distortion and typically exhibit third order intermodulation intercept points of 10-20 dBm. For many high performance applications, it is desirable to reduce the amount of intermodulation distortion and increase the level of the intercept point, thereby also substantially expanding the effective dynamic range of the mixer. In the past, it has only been possible to achieve such performance levels through the use of complex mixer circuitry requiring 10 to 30 separate circuit components to construct a single mixer which may be 10 to 20 times more expensive than conventional mixer devices.
It is therefore an object of the present invention to provide an improved mixer circuit characterized by low levels of intermodulation distortion and, in particular, having high second and third order intercept points.
It is another object of the present invention to provide an improved mixer circuit of moderate complexity which may be constructed from 5 to 15 separate components and which is relatively low cost in view of its performance.
It is a further object of the present invention to provide an improved mixer circuit having good balance and exhibiting therefore substantial isolation between the various mixer ports and also having a favorable conversion loss and a low noise figure.